The present invention relates to the drawing of a glass fibre, to be used as an optical wave guide, from a glass preform or gathering mould. Its object is to control and regulate the characteristics of the optical fibre.
It is known that it is necessary to master a certain number of parameters in order to produce a glass fibre which is able to serve as a light wave guide having constant optical transmission characteristics. The main parameters of the fibre formation operation, i.e. the drawing from a glass preform or gathering mould with a diameter much greater than that of the fibre are as follows:
the diameter of the preform, which must be as uniform as possible over the useful length thereof, PA1 the fibre diameter, which is linked with the drawing rate and which can easily be controlled during drawing, and PA1 the temperature of the fibre formation cone, i.e. the end of the preform from which the fibre emerges, said temperature determining the viscosity of the glass and by combination with the drawing rate the diameter of the fibre and that of the preform also determines the tensile stress.
However, said temperature is difficult to control in a precise manner. In the conventional fibre formation process the measurement of the temperature of the fibre formation cone within the furnace or oven is performed by means of an optical pyrometer by sighting through a window made in the furnace. Although the pyrometer may be accurate, the sighting or inspection window introduces a double source of error, because it firstly creates an asymmetry of the temperature gradient in the furnace and secondly a critical passage point for the light flux to be measured due to the frequent condensation of material on the transparent wall and the darkening which results therefrom.